A small-sized and highly-efficient permanent magnet motor (synchronous motor) has been widely used in a spindle motor of a HDD device and a motor drive unit of a fan, a pump and others.
However, since position information of a rotator of a motor is necessary for driving a permanent magnet motor, a position sensor therefor is required. In recent years, a sensorless control in which rotation and torque control of a permanent magnet motor is performed without using the position sensor has been widely prevalent.
By the practical use of the sensorless control, the reduction of the cost required for the position sensor (cost required for the sensor itself, wiring of the sensor and others) and the size reduction of the device can be achieved. In addition, since the sensor is unnecessary, such an advantage can be achieved that the use under adverse environmental conditions is possible.
At present, the sensorless control of a permanent magnet motor has adopted a method in which the induced voltage (speed electromotive force) generated by rotating a rotator of the permanent magnet motor is directly detected and used as the position information of the rotator to drive the permanent magnet motor, and a position estimation technique in which a rotator position is estimated and computed from an equation model of a target motor.
These sensorless controls have a significant problem. It is a position detection method at the time of low-speed driving. Since most of the sensorless controls put into practical use at present are based on the induced voltage generated by the permanent magnet motor, the sensitivity is decreased in a stopping or low-speed range where the induced voltage is low, and there is the possibility that the position information is buried in the noise. Various solutions have been suggested for this problem.
In the invention described in Japanese Patent Application Laid-Open Publication No. 9-327194 (patent document 1), the synchronous drive which is an open-loop control and the induced voltage detection are performed in combination in the stopping or low-speed range, thereby improving the drive performance in the low-speed range.
In the invention described in Japanese Patent Application Laid-Open Publication No. 2001-275387 (patent document 2), two phases of a three-phase stator winding are sequentially selected to apply a pulsed voltage, the induced voltage of a non-conduction phase induced by the pulsed voltage (transformer electromotive force in this case) is detected, and the position of a rotator is estimated from the voltage pattern thereof. This is because, since the saturation state of a magnetic circuit is changed depending on the position of the rotator, the induced voltage in accordance with the position is generated in the non-conduction phase. Therefore, in the invention described in the patent document 2, the position information can be acquired even in the complete stopping state.
In the invention described in Japanese Patent Application Laid-Open Publication No. 2003-189674 (patent document 3), the method of the patent document 2 is partially adopted at the time of activation (acceleration), and the acceleration is made steadily while confirming the position of a rotator.
In the invention described in Japanese Patent Application Laid-Open Publication No. 2000-232797 (patent document 4), the position information is obtained by detecting “neutral point potential” which is the potential at the connecting point of a three-phase stator winding. Although it requires an additional process for the extraction of the neutral point of the stator winding, since the position information can be obtained even when the three phases are all in the conduction state, it is possible to ideally drive the permanent magnet motor with the sine wave current.